There is an increasing demand for mobile communication devices capable of operating across a variety of different frequency bands. For example, there is an increasing demand for cellular or mobile telephones that can operate in multiple frequency bands. In such devices, transmit and receive filters may be employed for each transmit and receive frequency band. In practice, various types of acoustic resonators, including bulk acoustic wave (BAW) resonators, may be employed to construct filters for appropriate applications. In general, BAW resonators may include thin film bulk acoustic resonators (FBARs) and Solid Mounted Resonators (SMRs). BAW resonators may also be employed to construct oscillators, including tunable voltage controlled oscillators (VCOs) for some applications.
A typical implementation of an acoustic resonator comprises a piezo-electric layer (e.g., a layer of piezoelectric material) disposed between two electrically-conductive (e.g., metal) electrodes.
FIG. 1 shows a cross-section through an exemplary acoustic resonator 10 which comprises a piezo-electric layer, which will be referred to below as piezoelectric layer 12, disposed between a first, or bottom, electrode 11, and a second, or top, electrode 13. The designations top electrode and bottom electrode are just for definition purposes and do not represent any limitation with regard to the spatial arrangement and positioning of acoustic resonator 10.
If an electric field is applied between first electrode 11 and second electrode 13 of acoustic resonator 10, the reciprocal or inverse piezoelectric effect will cause acoustic resonator 10 to mechanically expand or contract, the case of expansion or of contraction depending on the polarization of the piezoelectric material. This means that the opposite case applies if the electric field is inversely applied between the electrodes 11 and 13. In the case of an alternating field, an acoustic wave is generated in piezoelectric layer 12, and, depending on the implementation of acoustic resonator 10, this wave will propagate, for example, in parallel with the electric field, as a longitudinal wave, or, as a transversal wave, transverse to the electric field, and will be reflected, for example, at the interface of piezoelectric layer 12. For longitudinal waves, whenever the thickness d of piezoelectric layer 12 and of the top and bottom electrodes equals an odd (1, 3, 5 . . . ) integer multiple of half the wavelength λ of the acoustic waves, resonance states and/or acoustic resonance vibrations will occur. Because each acoustic material has a different propagation velocity for the acoustic wave, the fundamental resonance frequency, i.e. the lowest resonance frequency FRES, will then be inversely proportional to weighted sum of all thicknesses of the resonator layers.
The piezoelectric properties and, thus, also the resonance properties of an acoustic resonator depend on various factors, e.g. on the piezoelectric material, the production method, the polarization impressed upon the piezoelectric material during manufacturing, and the size of the crystals.
FIG. 2 illustrates an example logarithmic input impedance response versus frequency for an example acoustic resonator. As shown in FIG. 2, in general the input impedance of an acoustic resonator will exhibit a sharp negative-going peak from a series resonance at a lower frequency fS, and a sharp positive-going peak from a parallel resonance at a higher frequency fP.
FIG. 3A shows a model of a bulk acoustic wave (BAW) resonator—for example a thin film bulk acoustic resonator (FBAR). The model of FIG. 3A is a modified Butterworth-Van Dyke model (MBVD) model. The frequency response of this model is a bandpass response, with frequencies below the passband being attenuated by the capacitors Cm and Co, and with frequencies above the passband being attenuated by the inductance Lm.
As shown in FIG. 3B, at series resonance, the device can be modeled by the series-resonant combination of Lm and Cm, in series with a parasitic resistance Rs. As shown in FIG. 3C, at parallel resonance, the device can be modeled by the parallel-resonant combination of Lm and Co, in parallel with a parasitic resistance Rp. The resistances Rs and Rp represent various heat losses and acoustic losses within the acoustic resonator.
As noted above, an acoustic resonator may be employed in an electrical filter, for example a radio frequency (RF) filter or a microwave filter. Acoustic resonators can be combined in various ways to produce a variety of filter configurations. An important characteristic of an RF or microwave filter is the insertion loss of the filter. In general it is desirable to provide RF and microwave filters with a low insertion loss (an ideal filter would have zero insertion loss). The insertion losses of an RF or microwave filter constructed with an acoustic resonator depends in turn upon the loss of the acoustic resonator, which is often expressed in terms of the resonator's quality factor, or Q, where a device with a higher Q has a lower loss than a device with a lower Q. Thus, all other things being equal, it is desirable to provide a filter with an acoustic resonator with a higher quality factor. As also noted above, an acoustic resonator may be employed in an oscillator. When an acoustic resonator is employed in an oscillator, the quality of the oscillator (e.g., phase noise) is affected by the quality factor of the acoustic resonator, where again—all other things being equal—it is desirable to provide an oscillator with an acoustic resonator with a higher quality factor. So, in general, it is desirable to improve the quality factor of an acoustic resonator. The quality factor of an acoustic resonator, in general, tracks the values of Rs and Rp in the models of FIGS. 3A and 3B which are representative of internal heat (ohmic and viscous-elastic) losses and acoustic (radiative) losses in the acoustic resonator. In particular, for a higher quality factor, the device should have a lower series resistance Rs and a higher parallel resistance Rp.
What is needed, therefore, are acoustic resonator structures that can provide an increased quality factor by exhibiting reduced internal losses.